In a slotted rigid coupling, one shaft to be coupled is inserted into a coupling portion provided with a slot, in which state the coupling portion is fastened by a fastening bolt, so that the shaft will be securely fastened coaxially with the coupling portion. A typical slotted rigid coupling comprises, e.g., a cylindrical coupling body, a round shaft hole of fixed inside diameter extending through the center of the coupling body, and a fastening bolt, as shown in FIG. 4 of patent document 1. A first slit is formed at a position partway along the coupling body, the first slit extending along the circumferential direction of the coupling body. A second slit is formed extending in the direction of the center axis of the coupling, the second slit extending from one end of the coupling body to the first slit. Once the shaft to be coupled has been inserted into the round shaft hole, the portion of the coupling body that is segmented by the second slit is fastened using the fastening bolt, whereby the shaft is securely fastened coaxially with the coupling body.
Such a rigid coupling is used in cases when, e.g. a motor output shaft is to be coaxially coupled with an input shaft for a reducer or the like. In patent documents 2 and 3, a shaft portion of a sun gear for a planetary gear drive is press-fitted and fastened from one of the open ends of a coupling, a motor output shaft is inserted from the other open end of the coupling, the other open end being provided with a slit, and the motor output shaft is securely fastened by a fastening bolt (not shown).